The present invention relates to a control method for controlling a movable member of an excavator. Besides, the present invention relates to an excavator comprising a control unit implementing such a control method.
The invention can be applied in construction equipment machines, such as mechanical shovels or drillers and any other type of excavator. Such excavator may be a tracked excavator comprising either a caterpillar track or wheels, and a cantilever member coupled to a rotating platform mounted on the caterpillar track.
The invention can also be applied to wheeled excavators and or to backhoe loaders. Although the invention will be described with respect to a mechanical shovel, the invention is not restricted to this particular construction equipment, but may also be used in other construction equipment machines.
WO13114451A1 discloses an excavator including several movable members and several electric actuators to actuate said movable members, several static brakes to lock said electric actuators, a command device to receive commands from an operator and a control unit to control said electric actuators and said static brakes.
The excavator of WO113114451A1 comprises a movable member for holding loads in service, an electric actuator with an electric motor, a brake for braking the electric motor in case of a risk of collision, and a motion sensor for delivering motion signals to the control unit.
However, in the excavator of WO13114451A1, the electric motor is continuously energized in order to immobilize the electric actuators thus inducing expensive electrical power consumption even during idle periods. Besides, the brake release causes some backlash in the electric actuator, which decreases the operator's comfort and may reduce the service life of some components of the excavator.
It therefore appears that, from several standpoints, there is room for improvement in the control methods for controlling a movable member of an excavator and in the excavator including a control unit implementing such a control method.
It is desirable to provide a control method which reduces or avoids the risk of backlash when the brake is released, while minimizing the electrical power consumption.
According to one aspect of the invention, a control method for controlling a movable member of an excavator includes a step of:
providing an excavator including at least:
a movable member configured to hold a load when said excavator is in service,
an electric actuator configured to actuate said movable member, said electric actuator comprising: i) an electric motor which is reversible and which is configured to apply a motor force on said electric actuator, and ii) a static brake configured to generate a brake force so as to brake said electric actuator,
a control unit configured to control said electric motor and said static brake, and
a motion sensor unit configured to detect a motion of said electric actuator and to send motion signals to said control unit.
Furthermore, said static brake is configured to generate an upper threshold brake force and said electric motor is configured to generate an upper threshold motor force, said upper threshold brake force being inferior to said upper threshold motor force.
The control method includes at least:
performing an immobilization step wherein said control unit controls said static brake to generate said upper threshold brake force,
performing a slippage detection step wherein said control unit checks said motion signals in order to detect whether said electric actuator is moving in a slippage direction despite said static brake generating said upper threshold brake force,
in case said control unit detects that said electric actuator is moving in said slippage direction, performing a motor energizing step wherein said control unit controls the power supply to said electric motor so that said electric motor generates a motor force substantially equal or superior to said upper threshold brake force in a direction opposite to said slippage direction, and
after start of said motor energizing step and in case said electric motor generates a non null motor force, performing a brake release step wherein said control unit releases at least partially said static brake.
Thus, such a control method allows a smooth release of the static brake even though the electric actuator carries a high load. Conversely, such a control method avoids the backlash when stopping a slippage of the electric actuator carrying a high load. Indeed, the electric motor is energized up to the upper threshold motor force before the static brake is released. Hence when the static brake is released, the electric motor already carries the load and thus substantially holds in place the electric actuator.
Besides, such a control method spares electrical power, since the electric motor can be shutdown when the static brake suffices to hold a static load, hence most of the time. Thus, such a control method permits to give in smoothly to high, excessive loads while the static brake might fail to stop the load (slippage) and hence to smoothly give in to the load.
By the provision of such an excavator arm, an advantage of such a control method is the reduction of the risk of backlash when the static brake is released, while minimizing the electrical power consumption, as compared to the excavator of WO13114451A1. Indeed, the electric motor can remain idle over long periods, which permits to reduce electric power consumption. Once the electric motor gets energized, it can hold the loads in lieu of the static brake.
Throughout the present application, the term “motion sensor unit” defines a device configured to electronically monitor the motions or movements of or within a component, for instance of a movable member. A motion sensor generally produces an electrical signal that varies as said component moves. Thus, such a motion sensor unit allows the control unit to monitor the motions of one or several electric actuator(s).
Throughout the present patent application, the term “detect a motion of said electric actuator” involves at least detecting a change in a relative position of two parts of the electric actuator, and detecting the direction of this change in a relative position.
The motion sensor unit may include any kind of motion sensor so as to detect any non null speed or any motion, hence any change in position, of said electric actuator. For instance, the motion sensor unit may include at least one position sensor, an acceleration sensor or a speed or velocity sensor. For instance the motion sensor unit may include a rotary encoder or shaft encoder or any other electro-mechanical device converting an angular position of the rotary motor shaft into an analogue value or digital code. The motion sensor unit may include a velocity sensor, for instance an inductive sensor.
Throughout the present application, the term “slippage” refers to a motion of the electric actuator despite the static brake generating the upper threshold brake force.
According to a variant, said control unit is an electronic control unit.
According to a variant, said control unit comprises a memory storing at least a dataset containing data identifying every electric actuator belonging to said at least one actuating set. Thus, such a memory permit to define the actuating sets prior to using the excavator, for instance depending upon the combined motions which will most likely be commanded by the operator.
According to a variant said static brake may be configured to generate a brake force on said electric motor. Alternatively, said static brake may be configured to generate a brake force on an intermediate component, which in turn transmits the brake force to said electric motor.
The control unit may include a variable-frequency drive configured to control at least one electric motor. Besides, the control unit may be configured to receive motion signals from said motion sensor unit in order to assess the state of the electric actuator, in particular to assess whether said electric actuator is immobile or still moving.
During the immobilization step, the control unit may completely stop the power supply to the electric motor. Alternatively, during the immobilization step, the control unit may maintain a predetermined power supply to the electric motor, for instance a low power supply.
During said brake release step, said control unit may totally release the static brake, such that the static brake generates no brake force. Alternatively, during said release step, the control unit may partially release the static brake, such that the static brake generates a low brake force.
According to an embodiment, during said motor energizing step, said control unit controls the power supply to said electric motor so that said electric motor generates a motor force superior to 80%, for instance superior to 100%, of said upper threshold brake force.
Thus, such a control method allows a very smooth release of the static brake even though the electric actuator carries a high load.
According to an embodiment, the control method further includes at least:
after completion of said brake release step, performing a motion detection step wherein said control unit checks said motion signals in order to detect whether said electric actuator keeps moving in said slippage direction,
in case said electric actuator is moving in said slippage direction, performing a check step wherein said control unit checks whether said electric motor is generating a motor force equal or superior to said upper threshold motor force, and
in case said electric motor is generating a motor force equal or superior to said upper threshold motor force, performing an overload relief step wherein said control unit controls said electric motor to generate said upper threshold motor force.
Thanks to this aspect of the invention, the risk of damaging the mechanical parts of the movable member are reduced or avoided.
According to an embodiment, the control method further includes:
in case said electric motor is generating a motor force inferior to said upper threshold motor force, performing an incremental step wherein said control unit increases said motor force,
performing a loop step wherein said motion detection step, said overload check step, said overload relief step and/or said incremental step are repeated until said electric actuator remains motionless during a predetermined period, and
in case said electric actuator has stopped moving during said predetermined period, performing said immobilization step.
The loop step can be repeated until the movable member is motionless. Then, the control unit controls the electrical motor so that an acceptable torque maintains the load motionless. If the torque is superior to the acceptable torque then motion will occur while torque is limited. Once motion subsides and torque remains acceptable then the immobilization step takes place.
Thus, such a control method allows limiting the slippage of the static brake while allowing slippage of the movable member via the electrical motor. Hence, such a control method allows using a static brake to brake the electric motor. Indeed, in case the movable member is subjected to an overload, namely a load superior to said upper threshold motor force, such steps allow to smoothly give in to an overload superseding the upper threshold motor force. Alternatively, such steps allow taking over control of a load larger than said upper threshold brake force but inferior to said upper threshold motor force, hence inferior to an overload.
Alternatively to the previous embodiment, when the load on said electric actuator increases, said control unit may operate another actuator in order to stop the slippage of said electric actuator.
According to an embodiment, said control unit further comprises a timer for counting said predetermined period, and wherein said predetermined period ranges from 1 s to 5 s.
Thus, such a predetermined period enhances the safety of the excavator, because it ensures that the movable member is completely stopped, hence not slipping, either by the motor force or the brake force.
According to an embodiment, wherein said control unit performs said motor energizing step in case said motion signals present a predetermined condition, for instance in case an amplitude of said motion signals supersedes a predetermined motion threshold.
Thus, unnecessary energizing steps are avoided, for instance when vibrations occur that generate small motion signals or noise.
According to a variant, the predetermined condition may be defined as a range of slippage amplitude. For instance, in case the electric motor has a reduction ratio of 25, the slippage amplitude may range from 37.5 degrees to 75 degrees. Alternatively, a low-pass filter may be applied on the motion signals.
According to an embodiment, during said motor energizing step, said control unit controls the power supply to said electric motor so that said electric motor generates a motor force ranging between 100% and 120% of said upper threshold brake force.
Thus, such a range allows the motor force to be superior to the upper threshold brake force.
According to an embodiment, said upper threshold motor force ranges between 100% and 300% of said upper threshold brake force.
Thus, such a range allows the upper threshold motor force to hold loads despite slippage occurring at the static brake.
According to an embodiment, said upper threshold brake force can range between 33% and 99%, for instance between 66% and 98%, of the maximum brake force. Throughout the present application, the term “maximum brake force” refers to the highest possible brake force that the static brake can generate.
According to a variant, said upper threshold motor force can range between 33% and 99%, for instance between 66% and 98%, of the maximum motor force.
Throughout the present application, the term “maximum motor force” refers to the highest possible motor force that the electric motor can generate.
According to an embodiment, said upper threshold brake force corresponds to a predetermined overload limit.
Thus, such an upper threshold brake force can hold any load applied during normal service conditions of the excavator.
According to an embodiment, said predetermined overload limit is set as a function of the mechanical strength of said movable member.
Throughout the present application, the term “mechanical strength of the movable member” refers to the yield strength of the movable member, hence to the force that would start the plastic deformation of the movable member.
According to an embodiment, said predetermined overload limit ranges between 25% and 80% of the mechanical strength of said movable member.
Thus, such a predetermined overload limit ensures that the control unit will give in to an overload without risking to break the structure of the movable member.
According to an embodiment, said movable member is selected from the group consisting of a tool configured to work on a site, an arm configured to move said tool, a boom configured to move said arm, a swing member configured to swing said boom, a drive member configured to displace said swing member with respect to a site ground and an offset member.
According to an embodiment, said motion sensor unit includes a motion sensor selected from the group consisting of an encoder coupled with said electric actuator, a motion detector and a system comprising at least two position sensors or two distant elements which are configured to cooperate in order to generate motion signals.
Thus, such a motion sensor unit provides the control unit with frequent, accurate motion signals.
According to an embodiment, said electric actuator is selected in the group consisting of linear electric actuators and rotational electric actuators.
According to a variant, said electric actuator can be a linear electric actuator and further comprise a converter configured to convert rotary motion into linear motion. A linear electric actuator usually comprises a linear actuating rod, which is linearly movable. The electric motor can apply a driving torque on the linear actuating rod, in which case the motor force is a motor torque. Likewise, static brake can usually apply a brake torque on the linearly movable rod, in which case the brake force is a brake torque.
According to a variant, said electric actuator can comprise a reversible mechanical linear actuator. For instance said electric actuator can comprise a ball screw, a roller screw or a buttress thread screw, the screw imparting translation to a linear actuator rod by a nut.
According to a variant, said electric actuator can further include an actuating device and a gearbox configured to transmit power from said electric motor to said actuating device.
According to another aspect of the invention, an excavator includes at least:
a movable member configured to hold a load when said excavator is in service,
an electric actuator configured to actuate said movable member, said electric actuator comprising: i) an electric motor which is reversible and which is configured to apply a motor force on said electric actuator, and ii) a static brake configured to generate a brake force so as to brake said electric actuator,
a control unit configured to control said electric motor and said static brake,
a motion sensor unit configured to detect a motion of said electric actuator and to send motion signals to said control unit,
wherein said static brake is configured to generate an upper threshold brake force and said electric motor is configured to generate an upper threshold motor force, said upper threshold brake force being inferior to said upper threshold motor force, and
wherein said control unit is further configured to:
perform an immobilization step wherein said control unit controls said static brake to generate said upper threshold brake force,
perform a slippage detection step wherein said control unit checks said motion signals in order to detect whether said electric actuator is moving in a slippage direction despite said static brake generating said upper threshold brake force,
in case said control unit detects that said electric actuator is moving in said slippage direction, perform a motor energizing step wherein said control unit controls the power supply to said electric motor so that said electric motor generates a motor force substantially equal or superior to said upper threshold brake force in a direction opposite to said slippage direction, and
after start of said motor energizing step and in case said electric motor generates a non null motor force, perform a brake release step wherein said control unit releases at least partially said static brake.
For instance, said static brake can be gradually and totally released during the brake release step.
Within the scope of the present invention, the afore-mentioned embodiments and variants can be considered either in isolation or in any technically possible combination.